Hand dryers are often installed in public washrooms as an alternative to paper towels.
There are three main types of hand dryer on the market: “warm-air” hand dryers, “high speed” hand dryers and “air-knife” hand dryers.
Warm air hand dryers are very well known. They are invariably low flow, low speed machines which rely on heating the air to promote an evaporative drying effect at the surface of the hand. Examples include the Model-A Series of hand dryers manufactured and marketed by World Dryer Corporation, which is illustrated in FIG. 1. The heated airflow is typically discharged through a single nozzle a and the drying action is a “hand-over-hand” action, requiring the user to rub the hands together under the nozzle a with the aim of encouraging the evaporative drying effect.
High speed hand dryers, as the name suggests, use high speed airflow (>80 m/s) to provide a momentum-drying effect at the surface of the hands. Examples include the Xlerator® hand dryer manufactured and marketed by Excel Dryer Inc. and the Airforce manufactured by World Dryer Corporation. Again the airflow is typically discharged through a single relatively large nozzle—or a cluster of smaller nozzles—and the mode of use is somewhat similar to the “hand-over-hand” action of the warm air dryer, with the hands being held or cupped together underneath the nozzle to dry them. However, instead of being evaporated, the majority of the water on the surface of the hands is instead driven or blasted from the hands by the high-momentum airflow. The airflow tends not to be heated, though waste heat from the motor may in some cases be used to heat the airflow to a degree.
The third general type of hand dryer is the air-knife hand dryer, examples of which include the Dyson Airblade range of hand dryers manufactured by Dyson (UK) Limited and the Jet Towel range of hand dryers manufactured by Mitsubishi Electric Corporation.
Air-knife hand dryers use an “air-knife”—effectively a sheet or curtain of fast-moving air—which moves across the surface of the hand and, as it does so, removes water mechanically by scraping or stripping the water from the surface of the hand.
In certain models of Dyson Airblade hand dryer—and also in the Mitsubishi Jet Towel range of hand dryers—two opposing, stationary air-knives are used: one for each side of the user's hand. The hands are inserted between the air-knives and then withdrawn slowly to effect the required relative movement between the hands and the air knives.
In the Mitsubishi arrangement—shown in FIG. 2—the air-knife is discharged through opposing rows of individual discharge apertures (only the rear row b is visible in FIG. 2): here, the individual jets combine to produce the air knife downstream of the discharge apertures.
In the Dyson arrangement—which is described in UK Patent No. GB2428569 (hereafter, GB2428569) and reproduced in FIG. 3—the air knives are instead discharged through narrow, continuous slots (only the rear slot c is visible in FIG. 1), each less than 1 mm wide. The slots are ergonomically designed to mirror the anatomic shape of the user's hands. Thus, the front slot is straight to reflect the relatively flat palms of the user's hands and the rear slot c incorporates a pair of concave portions intended to conform better to the slightly-rounded back of the user's hand.
Later models of Dyson Airblade hand dryer—one of which is described in European Patent No. EP2744380 (hereafter, EP2744380) and reproduced in FIG. 4—provide a single-sided air-knife drying action and the user dries the hands one side at a time. Again, the air-knives (one for each hand) are discharged each through a narrow, continuous slot e less than 1 mm wide.